A large number of various apparatuses are known for examining or testing elongated textile test materials such as a card sliver, roving, yarn or woven fabric for example. Depending on their application, they can be categorized into the two classes of laboratory test (offline) and testing during the production process (online). The apparatuses make use of the various known sensor principles, of which the capacitive measuring principle being of special interest in this case. A measuring circuit with a measuring capacitor which is arranged as a plate capacitor is provided. An electric alternating voltage is applied to the measuring circuit, through which an alternating electrical field is produced in the measuring capacitor. The test material is moved through the plate capacitor and is subjected to the alternating field. Dielectric properties of the test material are determined. The parameters of the test material such as mass per unit of length and/or material composition are determined from the dielectric properties. Examples for suitable measuring circuits and evaluation circuits for their output signals can be found in the specifications EP-0'924'513 A1, WO-2006/105676 A1 and WO-2007/115416 A1.
In order to enable the performance of precise measurements which are not influenced by external influences such as air temperature or air humidity, a compensation method is frequently applied. For this purpose, the measuring circuit contains a reference capacitor in addition to the actual measuring capacitor. It can be formed by adding a third capacitor plate for example which is arranged parallel to the two measuring capacitor plates and is electrically connected in series with them.
The measuring circuit should supply an output signal zero when an alternating voltage is applied without test material. Due to the various imperfections in real electric components, it is not sufficient in practice to arrange the measuring circuit symmetrically in order to obtain a zero signal without the test material. Each measuring circuit needs to be balanced individually for symmetrization. Symmetric balancing occurs in production by the manufacturer and optionally during maintenance by a service technician. For this purpose, the capacitance of at least one capacitance trimmer connected in parallel to the measuring circuit is usually changed. Trimming occurs manually with a suitable tool, e.g. a screwdriver. The known method of laser trimming is applied alternatively. In any case, the apparatus needs to be opened for the balancing. Such a manual balancing is laborious, time-consuming and expensive.